1. Field of the Invention
The invention relates to methods for manufacturing semiconductor devices having improved device performances, and, more particularly to methods for forming a relaxed SiGe film.
2. Background Description
The escalating requirements for ultra large scale integration semiconductor devices require ever increasing high performance and density of transistors. With device scaling-down reaching limits, the trend has been to seek new materials and methods that enhance device performance. One of the most direct methods to increase performance is through mobility enhancement. It has been known that stress or strain applied to semiconductor lattice structures can improve device performances. For example, an N type device formed on an biaxially strained (e.g., an expanded lattice) silicon substrate exhibits better device performances than other N type devices formed on a silicon substrate without strain (or the expanded lattice structure). Also, a P type device having longitudinal (in the direction of current flow) compressive strain exhibits better device performance than other P type devices formed on a silicon substrate without such strain. The P type device also exhibits enhanced performance with very large biaxial tensile strain.
Alternatively, it has been known that a device exhibits better performance characteristics when formed on a silicon layer (or cap) that is epitaxially grown on another epitaxially grown SiGe layer that has relaxed on top of the silicon substrate. In this system, the silicon cap experiences biaxial tensile strain. When epitaxially grown on silicon, an unrelaxed SiGe layer will have a lattice constant that conforms to that of the silicon substrate. Upon relaxation (through a high temperature process for example) the SiGe lattice constant approaches that of its intrinsic lattice constant which is larger than that of silicon. A fully relaxed SiGe layer has a lattice constant close to that of its intrinsic value. When the silicon layer is epitaxially grown thereon, the silicon layer conforms to the larger lattice constant of the relaxed SiGe layer and this applies physical biaxial stress (e.g., expansion) to the silicon layer being formed thereon. This physical stress applied to the silicon layer is beneficial to the devices (e.g., CMOS devices) formed thereon because the expanded silicon layer increases N type device performance and higher Ge concentration in the SiGe layer improves P type device performances.
Relaxation in SiGe on silicon substrates occurs through the formation of misfit dislocations. For a perfectly relaxed substrate, one can envision a grid of misfit dislocations equally spaced that relieve the stress. The misfit dislocations facilitate the lattice constant in the SiGe layer to seek its intrinsic value by providing extra half-planes of silicon in the substrate. The mismatch strain across the SiGe/silicon interface is then accommodated and the SiGe lattice constant is allowed to get larger.
However, the problem with this conventional approach is that it requires a multi-layered SiGe buffer layer that is very thick (e.g., a thickness of approximately 5000 Å to 15000 Å) to achieve misfit dislocations on its surface portion while avoiding threading dislocations between the SiGe layer and the silicon substrate layer, thereby achieving a relaxed SiGe structure on the surface of the multi-layered SiGe layer. Also, this approach significantly increases manufacturing time and costs. Further, the thick graded SiGe buffer layer cannot be easily applied to silicon-on-insulator (SOI). This is because for silicon-on-insulator the silicon thickness has to be below 1500 Å for the benefits of SOI to be valid. The SiGe buffer layer structure is too thick.
Another problem is that misfit dislocations formed between the SiGe layer and the silicon epitaxial layer are random and highly non-uniform and cannot be easily controlled due to heterogeneous nucleation that cannot be easily controlled. Also, misfit dislocation densities are significantly different from one place to another. Thus, the physical stress derived from the non-uniform misfit dislocations are apt to be also highly non-uniform in the silicon epitaxial layer, and this non-uniform stress causes non-uniform benefits for performance with larger variability. Further at those locations where misfit density are high, the defects degrade device performances through shorting device terminals and through other significant leakage mechanisms.
Therefore, there is a need for effective methodology for manufacturing a relaxed SiGe layer.